Method of annealing copper and its alloys



United States Patent 3,488,232 METHOD OF ANNEALING COPPER AND ITS ALLOYS Joseph Bernard Cotton, Sutton Coldfield, and David James Holmes, Birmingham, England, assignors to Imperial Metal Industries (Kynoch) Limited, Birmingham, Warwickshire, England, a corporation of Great Britain No Drawing. Filed Dec. 22, 1966, Ser. No. 603,726

Claims priority, applicatiggwclsrgeat Britain, Jan. 5, 1966,

Int. Cl. C23c 3/00 US. Cl. 14813.2 12 Claims ABSTRACT OF THE DISCLOSURE Cooper base alloys which are susceptible to red staining after surface oxidation during heat treating in an oxidizing atmosphere are treated by applying to the surface of the alloy, prior to heating in the oxidizing atmosphere, a film of an alkaline salt containing a metal-bearing anion such as an aluminate, stannate, zincate, tungstate or molybdate.

This invention relates to the heat treatment of copper and its alloys and, in particular, to a method of annealing zinc-containing copper-base alloys in oxidizing atmospheres.

It is common practice to anneal copper and its alloys, in strip or like form, by passing them through a furnace having an atmosphere of air. Although this results in the formation of a layer of oxide scale on the metal, the layer so formed is usually removable by pickling in dilute sulphuric acid. However, certain zinc-containing copperbase alloys, for example, 70/30, 60/40 and 65/35 copper/ zinc brasses do not always respond to pickling in sulphuric acid, areas of red cuprous oxide remaining after the treatment. This phenomenon is known as red staining.

These red stains withstand repeated pickling in sulphuric acid and are usually removable only by powerful cleaning agents such as chromic acid, i.e. a solution of sodium dichromate in sulphuric acid. The use of such materials is expensive and is not always successful, with the result that badly stained material may have to be remelted and processed.

The susceptibility of this class of alloy to red staining is, in part, a function of zinc content, the higher the zinc content the more susceptible the alloy. There are occasions, however, when the layer formed during annealing consists entirely of zinc oxide. When this occurs, the layer protects the underlying metal from further oxidation, but occasionally the zinc oxide flakes off during the cooling cycle which normally precedes a water-quenching operation. When this occurs, the metal is still sufiiciently hot to reoxidise and, since its surface is now depleted of zinc, copper oxides form. The new oxide film consists of a layer of black cupric oxide, which is soluble in the sulphuric pickling acid, and an underlying layer of cuprous oxide, which is insoluble in the said pickling acid and gives rise to red staining.

It is an object of the invention to provide a method of annealing copper-base alloys which inhibits or lessens the above-mentioned difficulties.

According to the invention, a method of heat treating a copper-base alloy which is susceptible to red staining comprises applying a film of an alkaline salt containing a metal-bearing anion to the surface of the alloy before it is heated to the heat treatment temperature.

The alkaline salt should be such that its aqueous solution (2% weight/volume) has a pH value of at least 9 and, preferably, 14. It has been found that salts whose solutions have pH values from 9 to 10 lessen the amount 3,488,232 Patented Jan. 6, 1970 of red staining, but that several salts whose solutions have a pH value of 14 inhibit the staining completely. Salts which completely inhibit red staining include alkali metal aluminate, stannate and zincate; those which lessen the staining include alkali metal tungstat'e, molybdate, silicate and borate.

The salt may be conveniently applied to the metal in the form of a solution by means of swabbing, brushing, dipping or spraying so as to produce a continuous film of liquid over the surface to be exposed to the annealing atmosphere. The solution is then evaporated to produce a layer of the said salt on the metal before it is annealed.

Preferably, the salt solution is applied to the warm metal to accelerate evaporation of the solvent, which is usually water. Should it be desired to treat metal which has been stored, it would be advisable to pre-heat the metal; but the treatment will most frequently be applied to strip or sheet metal as it issues from a rolling process when it is at an elevated temperature, e.g., -l40 C.

The salt solution may be of any suitable strength, but 2% Weight/ volume solutions have been found to be both effective and economical. A small amount of a wetting agent may be added to the solution to promote the formation of a continuous film of liquid on the surface of the metal.

Should it be desired to apply the method of the invention to a fabricated article, e.g., a metal cup, it may be difficult to obtain a satisfactory film of salt in re-entrant angles and the like. This is thought to be due to the presence of lubricant from the fabricating processes and may be overcome by applying the film of salt during fabrication of the article. This may be accomplished by using, as a lubricant, a soap-incorporating the salt, provided that the soap is sufficiently alkaline to prevent precipitation of the metal-bearing anion.

After annealing and water-quenching, the metal may be freed of its salt and oxide layers by pickling in sulphuric acid.

In order that the invention may be more fully understood, it will be illustrated, by way of example only, by the following experiment:

Hard rolled brass sheets of 70/30 copper/zinc composition were swabbed with an aqueous solution containing 2% weight/volume of sodium aluminate and 0.001% volume/volume of Lissapol (R.T.M.) wetting agent, using a soft mop. The sheets were then allowed to dry in a warm atmosphere, so as to produce an even layer of sodium aluminate on their surfaces. The sheets were then annealed by heating to 650 C. for 3% hours, after which time they were quenched by immersion in water at room temperature. The sheets were then pickled in 10% weight/ volume sulphuric acid and were found to be free of all forms of staining. Similar sheets which were treated in the same way, but not coated with aluminate were found to be red stained.

We claim:

1. In a process for heat-treating in an oxidizing atmosphere a copper-base alloy which is susceptible to red staining after surface oxidation, the improvement which comprises applying to the surface of the alloy prior to heating in said atmosphere, a film of an alkaline salt containing a metal bearing anion selected from the group consisting of aluminate, stannate, zincate, tungstate and molybdate.

2. A method as claimed in claim 1 in which the alkaline salt containing a metal-bearing anion is applied as an aqueous solution.

3. A method as claimed in claim 2 in which the alkaline salt containing a metal-bearing anion is applied as an aqueous solution containing 2% by weight of the salt.

4. A method as claimed in claim 2 in which the aqueous solution has a pH value at least 9.

5. A method as claimed in claim 2 in which the aqueous solution has a pH value of 14.

6. A method as claimed in claim 2 in which the aqueous solution has a pH value in the range 9-10.

7. A method as claimed in claim 2 in which the aqueous solution contains a wetting agent.

8. A method as claimed in claim 1 in which the film of salt is applied to said alloy during fabrication of the alloy into an article.

9. A method as claimed in claim 8 in which the salt is incorporated in a lubricant which is applied to said alloy during said fabrication.

10. A method as claimed in claim 9 in which the lubricant is a soap sufficiently alkaline to prevent precipitation of the metal-bearing anion.

11. A method as claimed in claim 2 in which the alloy to which the aqueous solution is applied is preheated,

12. A method of annealing a copper-base alloy containing 70% copper, 30% zinc in sheet form in which an aqueous solution containing 2% by weight of sodium aluminate and 0.001% by volume of wetting agent is applied to the surfaces of the sheet, the solution is evaporated to form a coating of sodium aluminate on the sheet, and the sheet is annealed at 650 C. for 3% hours and quenched in water.

References Cited UNITED STATES PATENTS 2,286,745 6/1942 Liebhafsky 148-132 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R. l48-28, 20.6 

